Anyone using an R8?

[neuroanatomist]

I was under the impression the EFCS reduced, but didn't eliminate, the jello effect. Still, if the readout speed is just 4x the time it take for the shutters to cross the frame, it's likely fine.

Nope, as far as rolling shutter goes EFCS is the same as mechanical shutter. With mechanical shutter, the shutter opens (first curtain) to begin the exposure and closes (second curtain) to end it. With EFCS, the sensor begins the exposure (that happens all at once), and the mechanical shutter ends it. In both cases, the sensor is read out with the shutter closed, after the exposure has ended. Readout speed doesn’t matter.

As I said, technically mechanical shutter can also suffer from a rolling effect with a subject moving fast enough, and thus EFCS can also. So from that standpoint, EFCS does reduce but not eliminate the jello effect...it reduces to the same magnitude/problem as with a mechanical shutter.

As a side note, EFCS also eliminates the shutter shock sometimes observed with mechanical shutter so in some ways it's the best of both worlds. Shutter shock comes from the vibration of the first curtain opening. Of course, the second curtain closing also causes vibration but in that case the exposure was already ended by that curtain so there's no more light hitting the sensor as the vibration occurs. However, I recently learned that the R7 has a problem with shutter shock even with EFCS...the vibration is strong enough to affect the next shot in a burst.

I doubt you have any EF-S lenses kicking around to judge the quality on the 9.4MP crop mode images, which really would only affect the 10-18mm.

Nope, sorry. I've only owned two EF-S lenses (the 17-55/2.8 that was the first lens I bought with my T1i/500D, and the 10-22), and I sold both when I sold my 7D, after getting the 1D X.